1. Field of the Invention
The present invention pertains to a toner concentration measuring device that illuminates developer comprising toner and carrier and detects the toner concentration in the developer based on the reflected light from the developer.
2. Description of the Related Art
In a developing system that is used in an electrophotographic image forming apparatus and that uses two-component developer comprising toner and carrier, in order to ensure a certain image density, the percentage of the toner in the developer (hereinafter the `toner concentration`) must be measured and the toner must be added to the developer in accordance with the result of said measurement.
For this purpose, an optical toner concentration measuring device has been known that illuminates the developer inside the developing system via a transparent plate and detects the toner concentration based on the reflected light from the developer.
As an improved version of this optical toner concentration measuring device, a device has been proposed in which the transparent plate is made of a material that becomes charged, through its contact with the developer (the carrier contained in the developer, in particular), with the same polarity as the charge polarity of the toner, such that the toner will be electrically repelled and prevented from adhering to the transparent plate.
When the developer, particularly the carrier, is still new, the transparent plate becomes sufficiently charged through its contact with the carrier to prevent toner adherence. However, when the carrier becomes old, its charging capacity with respect to the toner and the transparent plate (its charge-providing capability) decreases, and it can no longer effectively prevent the toner from adhering to the transparent plate. Consequently, the problem arises that the toner concentration cannot be accurately measured, which leads to a loss of the control necessary to maintain appropriate image density.